Injection moulding apparatus

ABSTRACT

A mould for an injection moulding apparatus, the mould including at least two mould parts which mate to form the mould cavity and which separate to permit removal of the moulded article, each mould part including a relatively rigid body having an abutment face which abuts with an abutment face of the other mould part when said parts mate, the body of at least one mould part including a recess in its abutment face which is filled with a resilient material defining a portion of said mould cavity.

This application is a continuation of application Ser. No. 07/248,068,filed Sept. 23, 1988, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an injection moulding process, articlesmoulded by the process, and an injection moulding apparatus.

2. Description of the Related Art

Injection moulding of thermoplastics to create a product requires thecreation of a separable mould. The mould normally comprises two mouldparts and these mould parts may incorporate movable core inserts for themoulding of intricate shapes having undercuts.

Generally, acute undercuts in a moulding are not possible without acomplicated mould construction which results in expensive tooling costs.In addition such moulds can take many months to produce. Even if a mouldis produced for providing undercuts, the number of undercuts providedwill be restricted such that, in the main, only straightforwardgeometrical snapes are produced using these techniques.

These drawbacks become more apparent when articles of non-geometricrandom shape are to be moulded. Such articles may for instance be modelsof animal shapes.

Heretofore, injection moulded models of animal shapes made from plasticshave lacked realistic detail in shape due to the inability ofconventional injection moulding techniques to produce many randomlyshaped undercuts and fine detail. In addition since the mould cavity inthe injection mould parts is defined by metal it has to be shaped usingerosion techniques such as spark erosion or pantography and consequentlyit is not possible to accurately recreate the fine detail of anoriginal, artistically, created sculpture.

In addition it will be appreciated that because of the high cost ofcreating injection moulds it is not economically practical to produce arelatively small number of mouldings of one particular model and issimilarly not economically practical to produce a large number of mouldsfor creating a wide variety of such models.

SUMMARY OF THE INVENTION

It is a general aim of the present invention to provide a mould for aninjection moulding apparatus which is capable of producing articles ofeither a geometric or a non-geometric random shape, in particularreplicas of sculptures such as miniatures or models of characterfigures.

According to one aspect of the present invention, there is provided amould for an injection moulding apparatus, the mould comprising at leasttwo mould parts which made to form the mould cavity and which separateto permit removal of the moulded article, each mould part comprising arelatively rigid body having an abutment face which abuts with anabutment face of the other mould part when said parts made, the body ofat least one mould part having within its abutment face a resilientmaterial defining a portion of said mould cavity.

Preferably the resilient material projects above the abutment face ofthe mould part so that on closing of the mould parts the resilientmaterial is placed under compression.

Preferably all mould parts include a recess in its abutment face eachcontaining a resilient mould insert. The resilient mould insert forinsertion in the or each recess maybe in the form of a block directlyinsertable into the or each recess or may be a block located on a rigidsupport body for insertion into the or each recess. The support body maybe a backing plate to which the block of resilient material is attached.Alternatively the support body may contain a recess formed in anabutment face which is contiguous with the abutment face of the mouldpart body when the support body is located within the mould part.Preferably the resilient material is a thermoset elastomer. Theelastomer may be a composition based upon a synthetic rubber or anatural rubber.

The recess may be filled by moulding the resilient material in situ orby inserting a preformed block of resilient material into the recess.

Location formations may be provided in selected positions about thecavity portion defined in the resilient material such that when themould parts mate the location formations co-operate to resist relativeslidable movement of the contacting surfaces of the resilient materialof respective mould parts.

According to another aspect of the present invention there is provided aprocess for creating resilient mould inserts for an injection mouldcomprising the steps of placing an article of which subsequent mouldingsare to be produced between separable cast mould parts containing amouldable material which is curable to form a resilient material,closing the cast mould parts so as to press the article into themouldable material contained within the cast mould parts so as to createan impression of the article in said mouldable material, curing saidmouldable material to form said resilient material so as to retain saidimpression and then separating the mould parts to remove said article.

Attachment means, such as a rigid backing plate or bolts, forsubsequently attaching the mould insert into the injection mould may belocated within the cast mould parts so as to be attached to theresilient material during the curing process. It is envisaged that suchattachment means may be secured to the resilient material after removalfrom the cast mould. It is also envisaged that the cast mould itselfcould be used as attachment means in which case the resilient materialis retained within the cast mould part after the curing process.

Thereafter the resilient mould inserts can be fitted to an injectionmoulding machine and used to produce replicas of said article.Preferably said mouldable material is a composition based upon asynthetic rubber or a natural rubber and which on the application ofheat is cured to define said resilient material. Preferably saidmouldable material is reclaimed rubber originating from types. Curing ofthe rubber is achieved by maintaining the cast mould parts, whilstclosed, at an elevated temperature for a sufficient length of time toenable vulcanisation of the rubber to occur.

According to another aspect of the present invention there is provided aprocess for producing injection moulding comprising forming an injectionmould as defined above and injecting a moulding material into said mouldto produce a moulding.

In view of the resiliency of said resilient material and the resiliencyof the plastics material used for creating-the moulding, the mouldedarticle may have acute undercuts which do not prevent the article beingremoved from the mould cavity.

In addition fine detail of the original article is recreated by themould since the mould cavity is formed as an impression of the originalarticle.

In view of the simplicity of forming an injection mould according to thepresent invention it is possible to create a high quality ofreproduction for geometric or non-geometric shapes very economically. Inparticular the present process thereby makes it economically practicalto produce high definition moulds for producing small quantities ofreplicas of an original model and also to produce a wide variety of suchmoulds. Large volumes of such replicas can also be produced simply byreplacing worn out moulds.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of a process of producing a mould according to the presentinvention is hereafter described with reference to the accompanyingdrawings, in which:

FIG. 1 is shown as a schematic side section view of a pair of mouldparts shown in an open position;

FIG. 2 is a similar view to FIG. 1 showing the mould parts closed so asto mate;

FIG. 3 is a schematic end view of a mould platen in an injectionmoulding machine;

FIG. 4 is a schematic perspective view of a pair of separated mouldinserts according to one embodiment of the invention;

FIG. 5 is a schematic perspective view similar to FIG. 4 showing asecond embodiment of the invention; and

FIG. 6 is a schematic part cross-section through opposed mould platensillustrating mould inserts in situ.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIGS. 1 and 2 there is shown a pair of cast mould parts 12, 14 eachof which comprises a relatively rigid body 16. Each part 12, 14 includesan abutment face 17 which are in mutual contact when the mould partsmade and each abutment face 17 is provided with a recess 18 which isfilled with a mouldable material 20 which is curable to form a resilientheat stable material. A rubber composition based upon rubber reclaimedfrom tyres has been found suitable for moulding thermoplastics such aspolypropylene.

The out surface of the mouldable material 20 is arranged to besubstantially contiguous with the abutment face 17 and prior to creatingan impression is preferably coated with a release agent such as talcumpowder.

An original sculpture or article 30 from which replicas are to be madeis positioned between the mould parts 12, 14 and these are then closedby, for instance, a press (not shown). Closing of the cast mould partscauses the article 30 to be pressed into the opposed faces of themouldable materials and causes an impression to be made therein. Due tothe flow characteristics of the mouldable material a natural separationline around the periphery of the article is achieved between the opposedsurfaces of the mouldable materials.

The closed cast mould parts are now heated to cause the rubber compoundto vulcanise and thereby form a heat stable resilient material. Themould parts are now separated and the article is removed to leave amould casting 50 (see FIGS. 3 and 4).

The bodies of resilient material 28 may now be removed from the castmould parts 12, 14, thereby defining a pair of mould inserts 100, inblock form, formed from the resilient material. The pair of mouldinserts 100 can be directly mounted into opposed mould platens 40 (onlyone of which is shown in FIG. 3) of an injection moulding machine.Attachment means, for example bolts 101 (FIG. 4) or a rigid backingplate 110 (FIG. 5) may be received to the resilient material prior toinsertion of the mould insert into the mould platens 40. The backingplate 110 may include one or more bolts 111.

Advantageously the attachment means are located with the cast mouldparts so that the attachment means become secured to the resilientmaterial during the curing process.

It is envisaged that the backing plate 110 may be attached to theresilient material after casting, for instance by bonding.

Preferably, as schematically illustrated in FIG. 6, the resilient mouldinserts 100 when located within the opposed platens 40 are positionedsuch that the separation surfaces 120 of the mould inserts 100 arelocated slightly above the abutment faces 41 of the platens 40.Accordingly when the mould platens 40 close, the opposed surfaces 120 ofthe mould inserts initially contact one another and then are placedunder a resilient loading caused by compression of the resilientmaterial as the platens reach their fully closed position. The resilientloading on the opposed surfaces 120 resists flashing occuring during theinjection moulding process.

As illustrated in FIG. 2, it is envisaged that location formations, suchas pegs 25, may be spaced about the mould cavity such that when themould inserts 100 are closed the location formations co-operate toresist relative slidable movement of the opposed surfaces 120.

As illustrated in FIG. 3, it is envisaged that the cast mould parts canbe used as attachment means. In this case the opposed surfaces 120 ofthe resilient material are substantially contiguous with the abutmentfaces 17 of the mould parts.

The body 16 of each mould part has an outer shape complementary to thatof a location recess formed in a platen 40 of an injection mouldingmachine. Accordingly the body 16 can be accurately located in the platen40 whilst being easily removable to facilitate replacement of the mouldinserts.

The platen 40 shown in FIG. 3 includes radially extending channels 41along which plastics is injected. Each body 16 has a channel 42 formedtherein for communication with a respective channel 41. After curing andseparation of the mould parts 12, 14 a channel (not shown) is cut intothe resilient material for feeding plastics material from the channel 42of the body to the mould cavity portion formed therein.

When casting the resilient inserts the size of each recess 18 of thecast mould parts and the amount of mouldable material contained thereinis chosen to provide sufficient mouldable material to provide anadequate impression. Preferably in order to assist heat dissipationduring the moulding process the amount of mouldable material is chosento be minimum to obtain the above criteria. In addition it is envisagedthat the bodies 16 may be provided with ducts for coolant fluid whichcommunicate with coolant ducts formed in the supporting platen 40.

In addition, the resilient material at least in the vicinity of themould cavity is preferably arranged to be of a minimum thickness so asto resist distortion of the mould cavity arising from fluid pressure ofthe moulten plastics during the injection moulding process.

In order to assist heat dissipation the mouldable material may beadapted to improve its heat conductivity. For instance the chemicalcomposition of the mouldable material may be adjusted to maximise itsheat conductivity and/or the mouldable material may include particles ofa good heat conductor such as metallic particles dispersed therein. Inaddition, or as an alternative, coolant conduits, such as metallicpipes, carrying a coolant may extend into the recess 18 so as to bepartly or wholly surrounded by the mouldable material. Furthermore,blocks of suitable metals may be located within the mouldable materialto act as heat sinks. These blocks may be in direct contact with thebody 16 to thereby provide a good path of heat conduction from themouldable material and into the body 16.

We claim:
 1. A mold for a thermoplastics injection molding machine,comprising:a first mold part having a relatively rigid body portion andan abutment face within said rigid body portion; at least one additionalmold part having a relatively rigid body portion and an abutment facewithin said rigid body portion of said additional mold part which mayabut said abutment face of said first mold part; said relatively rigidbody portion of said first mold part further having a recess within itssaid abutment face and a mold insert being made of resilient materialand having a contact face and positioned within said recess and saidcontact face projecting above said abutment face such that when thefirst and at least one additional mold part are positioned against oneanother the contact face of said mold insert first contacts the abutmentface of said additional mold part and is compressed by said additionalmold part and, upon further closing, the abutment face of said firstmold part abuts the abutment face of said additional mold part and amold cavity is formed by the mold insert of the first mold part, saidmold cavity being suitable for forming a molded article bythermoplastics injection molding and when the first and at least oneadditional mold part are moved away from one another they permit removalof said molded article; and further wherein when said molded article isremoved from said mold insert, a part of said mold insert is moved bysaid molded article with respect to said relatively rigid body of saidfirst mold part and once said molded article is removed from said firstmold part, said part of said mold insert substantially returns to itsoriginal position with respect to said relatively rigid member of saidfirst mold part, thereby enabling said molded article to have acuteundercuts; an injection means for injecting thermoplastics fluids underpressure; and an inlet channel open at one end to said injection meansand open at the other end to said mold cavity.
 2. A mold according toclaim 1 wherein said each additional mold part has a recess within itsrespective abutment face and a mold insert positioned within each saidrecess such that said mold inserts form said mold cavity when said moldparts abut one another, each said mold insert being made up of saidresilient material and having a contact face which projects above thecorresponding abutment face so that when said mold parts are positionedin contact with one another, the contact faces are compressed againstone another and, upon further closing, the abutment faces abut oneanother and further wherein when said molded article is removed fromeach said mold insert, a part of each mold insert is moved by saidmolded article with respect to each said relatively rigid body of eachsaid additional mold part to permit removal of said molded article andonce said molded article is removed from each said additional mold part,each said part of each said mold insert substantially returns to itsoriginal position with respect to each said relatively rigid member ofeach said additional mold part.
 3. The mold according to claim 1 whereinthe resilient material is a thermoset elastomer.
 4. The mold accordingto claim 2 wherein the resilient material is a thermoset elastomer. 5.The mold according to claim 3 wherein the elastomer is a compositionincluding natural rubber.
 6. The molding according to claim 3 whereinthe elastomer is a composition including synthetic rubber.
 7. The moldaccording to claim 4 wherein the elastomer is a composition includingnatural rubber.
 8. The mold according to claim 4 wherein the elastomeris a composition including synthetic rubber.
 9. A mold according toclaim 2 wherein each said mold insert of each said recess is defined bya block of said resilient material which may be directly inserted intoeach said recess.
 10. A mold according to claim 9 wherein each saidblock of said resilient material has attachment means for securing eachsaid block into each said recess.
 11. The mold according to claim 2further comprising a location formation means for resisting slidablemovement of said abutment faces relative to one another, said locationformation means positioned on said contact faces of said mold parts.